1. Field of the Invention
The invention is related generally to the field of interpretation of reservoir monitoring using time-lapse seismic measurements. More specifically, the present invention is directed towards a novel configuration of sources and receivers for providing improved tomographic reconstruction of reservoir images.
2. Background of the Art
Geophysical surveys are used to discover the extent of subsurface mineral deposits such as oil, natural gas, water, sulphur, etc. Geophysical methods may also be used to monitor changes in the deposit, such as depletion resulting from production of the mineral over the natural lifetime of the deposit which may be many years. The usefulness of a geophysical study depends on the ability to quantitatively measure and evaluate some geophysical analogue of a petrophysical parameter that is directly related to the presence of the mineral under consideration.
Conventional crosswell seismic imaging typically utilizes a pair of boreholes in proximity to a reservoir of interest. In the first of these boreholes, a seismic source is deployed for emitting seismic energy into the region of interest. For reasons discussed below, swept frequency sources are commonly used. The source is sequentially moved through a series of positions within the first borehole and a seismic signal is generated at each position. The seismic energy passes through the subterranean formation of interest to the second one of the pair of boreholes. A receiver array is typically deployed within the second borehole and, like the seismic source, the receiver array is moved through a series of positions within the second borehole. By transmitting a signal from each source position in the first borehole and receiving data from each source position at each receiver position in the second borehole, a seismic crosswell data set is generated.
Referring to FIG. 1, crosswell imaging is typically performed using a crosswell imaging system which is generally indicated by the reference numeral 10. In system 10, seismic energy 12 is transmitted through a subsurface region 14 of the ground using a source 16 which is positioned in a first borehole 18. A movable seismic source moved to locations S1 . . . Sn is used to generate seismic waves denoted by 12 that propagate from the first borehole 18. The seismic waves are detected in the second borehole 24 by suitable detectors. Commonly, a receiver array 22 comprising receivers 23a, 23b . . . 23e. is used for the purpose. The receiver array may be moved to positions denoted by R1 . . . Rs. Exemplary raypaths 28a . . . 28e and 30a . . . 30e show the raypaths for the seismic waves propagating from various source locations to the different receivers. The objective of seismic tomography is to measure the travel times for a plurality of source-receiver combinations and from the known geometry of the source and receiver locations, determine the velocity field for the propagating waves in the region 10. Methods for inverting the measured travel times to obtain the velocity field are known in the art.
In reservoir monitoring, the region 10 includes a reservoir from which hydrocarbons are recovered. In natural recovery of hydrocarbons as well as in secondary recovery wherein a fluid is injected from an injection well into the reservoir, there is a continuing replacement of one fluid by another in the reservoir. In addition, there may also be changes in the pressure of the fluids in the reservoir. Fluid substitution as well as pressure changes are known to cause changes in seismic velocities. Hence by monitoring changes in the velocity field, some inference can be made about changes in the fluid distribution in the reservoir and/or pressure changes therein. This process is called time-lapse tomography.
Typically, downhole seismic sources are swept frequency sources. The reason why swept frequency sources are used is that they are low power sources that are less likely to cause damage to boreholes than impulsive, high-power sources. A number of other low power seismic sources have been developed to enable the acquisition of crosswell seismic data. These include resonators, piezoelectric transducers, and magnetostrictive transducers. Other sources such as explosive or implosive sources, downhole airguns, and sparkers generally have higher power output and are more likely to cause damage to the wellbore. This low-power limitation restricts utilization of crosswell seismic tomography to situations in which the interwell distance is relatively short (i.e., no more than about 1,000 feet).
Another limitation on the use of crosswell seismic tomography is the substantial costs associated with preparing the wells for deployment of downhole seismic sources and receivers. In most cases, the diameters of the downhole seismic sources and receivers are too large to fit into the production tubing used to convey fluids from the reservoir to the surface. Therefore, at most sites, the production tubing must first be removed from the wells in order to conduct a crosswell survey and then be reinstalled following completion of the survey. Associated with the removal of production tubing is the cost of shutting down production for the seismic study.
Blakeslee (U.S. Pat. No. 5,481,501), the contents of which are fully incorporated herein by reference, discloses a method in which no sources are used in the wellbore. The method taught by Blakeslee is illustrated in FIG. 2 which shows two wells, well A and well B, extending downwardly into the earth 110. A seismic source s is located on the surface 112 of the earth 110, substantially in line with, but not between, wells A and B. A plurality of downhole seismic receivers, a1 . . . an. Preferably, a plurality of downhole seismic receivers, b1 . . . bn. In order to simulate crosswell data between well A and B, the seismic source s is activated to generate a seismic signal which propagates through the subsurface formations and is recorded by each of the first well seismic receivers a1 . . . an. and each of the second well seismic receivers b1 . . . bn. The resulting seismic data are then processed to yield information regarding the interwell region between wells A and B.
The method used by Blakeslee uses Fermat's principle to a ray traveling from the source s to a receiver such as bk in FIG. 2. If the velocity field between the wells A and B is known, then the travel time from each of the positions a1 . . . an; to the receiver bk can be calculated. From Fermat's principle, the measured traveltime from s to bk, denoted by Ts,bk is then given as
 Ts,bk=Minj(Ts,aj+Taj,bk)  (1)
A similar reasoning applies to the case shown in FIG. 3 where the well B lies between the well A and the source s. For FIG. 3, the traveltime Ts,ak from the source s to a receiver ak is given asTs,ak=Minj(Ts,bj+Tbj,ak)  (2)The method of Blakeslee assumes a velocity field, checks to see if eqs.(1)-(2) are satisfied for all receiver positions, and if they are not satisfied, iteratively alters the velocity field until the equation is approximately satisfied within some error bounds.
By using the method of Blakeslee, there is no necessity for using downhole sources. Consequently, it is possible to use conventional surface seismic sources for obtaining data that can be used for tomographic reconstruction of a subsurface region.
One of the problems with crosswell tomography (real or simulated) is that the ray paths cover only a limited range of angles and there is a serious lack of near vertical raypaths. The effect of the limited aperture is that the tomographic reconstruction may have low vertical resolution. There is a need for a method of obtaining data for tomographic analysis that does not need downhole sources while, at the same time, not suffering from poor vertical resolution. The present invention satisfies this need.